1. Field of the Invention
The present invention relates generally to automatic throttle control systems for aircraft and more specifically to an improved automatic gain control arrangement for such systems whereby desirable operation is achieved under all operating modes including climb, descent, steady state cruise, and approach modes of operation.
2. Description of the Prior Art
In the past, automatic throttle control systems, especially those for large aircraft used by commercial airlines and military transport groups, were employed primarily during the approach to landing mode where tight flight path control is required and during which very high throttle control gains are required to provide fast response to air speed changes, such as produced by gusts, wind shear, and the like. Later, autothrottle control usage was extended to take off and initial climb out and was sometimes used during the let down modes but still not generally used during cruise modes. More recently, and as represented in the aforementioned Miller U.S. Pat. No. 3,691,356, automatic control of the aircraft throttles has been extended to use in all modes, including the climb-to-cruise altitude, extended cruise, long-term let down, holding pattern, extended approach, final approach, and go-around modes.
Normally, the aircraft aerodynamic configuration that is determines in part by the degree of extension of the flaps, slats, and other aerodynamic surfaces will differ during most of these various operating modes. Clearly, an autothrottle system with a fixed gain characteristic is far from desirable and usually unacceptable for such operation. For example, the high gain required for tight air speed control during let down, or approach modes will result in undesirable throttle activity during the cruise modes where tight control of air speed is, in fact, not required. Stated another way, during the cruise mode, it is desirable to minimize throttle activity and the degree of engine thrust variations at the expense of tight air speed control, especially in the presence of turbulence. However, there are circumstances during cruise in which it is desired to tighten the autothrottle system response, for example, when the pilot desires to change his air speed and when flight path changes are made, as when a push-over is effected during cruise to reach a lower altitude. In considering such needs, the requirement must be borne in mind that maximum gain must be provided at lower air speeds, particularly at approach to landing air speeds.
As disclosed in the Miller U.S. Pat. No. 3,691,356, the full time autothrottle control system is made practical largely because of an engine protection control disclosed therein as the thrust rating control. This control overrides autothrottle control in the air speed control loop and thus prevents engine overboost. The thrust rating computer generates a parmeter, such as the speed of the low speed rotor normally attached to the fan, indicative of maximum engine thrust performance under the existing operating conditions of the engine.
In the aforementioned Schloeman application, now U.S. Pat. No. 3,908,934 the reduction of engine throttle activity during cruise is accomplished generally by reducing the system gain as a function of indicated air speed whereby, at high air speeds, the system gain is low as is desired for reduced throttle activity. To provide the desired higher response to air speed changes commanded by the pilot or required during flight path changes, the gain is increased as a function of air speed error, that is, the difference between the actual and commanded air speed, the maximum error or error limit being effective above some predetermined air speed. While this gain control arrangement is generally satisfactory in most applications, it may not be entirely satisfactory in other applications for several reasons.
The response of large turbofan powered aircraft, in the high speed cruise mode, to throttle advance or retard commands is inherently sluggish as compared to the response to the same commands in the low speed and approach modes. This characteristic leads to operation as an underdamped control system at high air speeds. Reducing the system gain at high air speeds therefore tends to aggravate this inherent underdamped characteristic. Because aircraft automatic throttle control systems are inherently non-linear servo systems, the gain programming of such a system as a function of air speed error tends to reduce its apparent damping characteristic. Thus, a reduction of gain for small air speed errors will result in a very poorly damped control system. Furthermore, aircraft autothrottle control systems usually include substantial hysteresis effects associated with the physical coupling between the throttle controls levers and the engine fuel control apparatus and this hysteresis, coupled with an underdamped throttle level control servo loop response, may result in long period, low amplitude oscillations. The foregoing characteristics of an air speed error controlled gain program will render the autothrottle control system excessively responsive to turbulence which, in turn, results in increasing throttle activity without effectively reducing the unwanted air speed error.